drm/etnaviv: share a single cmdbuf suballoc region across all GPUs

There is no need for each GPU to have it's own cmdbuf suballocation
region. Only allocate a single one for the the etnaviv virtual device
and share it across all GPUs.

As the suballoc space is now potentially shared by more hardware jobs
running in parallel, double its size to 512KB to avoid contention.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Guido Günther <agx@sigxcpu.org>